Until fairly recently, the preferred, indeed the only means by which to display information in the electronic medium was to use a video monitor including a cathode ray tube ("CRT"). CRT technology has been well known for over 50 years, and has gained widespread commercial acceptance in applications ranging from desktop computer modules to home televisions and industrial applications. CRTs are essentially large vacuum tubes having one substantially planar surface upon which information is displayed. Coated on the inside of the CRT planar surface is a layer of phosphors which respond by emitting light when struck by electrons emitted from the electron gun of the CRT. The electron gun is disposed in an elongated portion which extends away from the inside of the CRT display surface.
While CRTs are widely used in numerous applications, there are several inherent limitations to the application of CRT technology. For example, CRTs are relatively large and consume a great deal of energy. Moreover, as they are fabricated of glass, the larger the display surface, the heavier the CRT. Given the need for the electron gun to be spacedly disposed from the phosphors on the surface of the display surface, CRTs have a substantial depth dimension. Accordingly, CRTs have little use in small and portable applications, such as handheld televisions, laptop computers, and other portable electronic applications which require the use of displays.
To answer the needs of the marketplace for smaller, lighter, more portable display devices, manufacturers have created numerous types of flat panel display devices. Examples of flat panel display devices include active matrix liquid crystal displays (AMLCD's), plasma displays, and electroluminescent displays. Each of these types of displays has use in a particular market application, though each is accompanied by various limitations which make them less than ideal for certain applications. Principal limitations inherent in devices such as AMLCD's relate to the fact that they are fabricated predominantly of inorganic semiconductor materials by semiconductor fabrication processes. These materials and processes are extremely expensive, and due to the complexity of the manufacturing process, cannot be reliably manufactured in high yields. Accordingly, the costs of these devices are very high with no promise of immediate cost reduction.
One preferred type of device which is currently receiving substantial research effort is the organic electroluminescent device. Organic electroluminescent devices ("OED") are generally composed of three layers of organic molecules sandwiched between transparent, conductive and/or metallic conductive electrodes. The three layers generally include an electron transporting layer, an emissive layer, and a hole transporting layer. Charge carriers specifically, electrons and holes, are generated in the electron and hole transporting regions, respectively. Electrons are negatively charged atomic particles and holes are the positively charged counterparts. The charge carriers are injected into the emissive layer, where they combine, emitting light. OED's are attractive owing to their thin profile, light weight, and low driving voltage, i.e., less than about 20 volts. Hence, they have a potential application as full color flat emissive displays.
One of the serious drawbacks attributable to OED's has been the difficulty encountered in achieving full color, i.e., red, green, blue ("RGB") display devices. This is owing to the fact that different organic materials will emit light at different wavelengths, and hence, different colors. For example, finding EL materials which provide outstanding red and blue colors with high color purity has heretofore been extremely difficult with the materials that have been available.
In view of the fact that emissive materials that can provide the desired colors have been difficult to identify, routineers in the field have attempted to augment the OED's with other devices so as to achieve the desired colors. For example, in U.S. Pat. No. 5,015,999 there is disclosed an OED device which emits radiation in the ultraviolet portion of the electromagnetic spectrum. This ultraviolet radiation is then passed through a filter which fluoresces blue light in response to the ultraviolet light generated by the OED. Approaches of this type have several limitations to their successful implementation. First, and foremost is the fact that there is not commercially available any OED which reliably emits radiation in the ultraviolet region. Moreover, devices such as those described in the '999 patent do not appear to be able to provide RGB display devices.
U.S. Pat. No. 5,126,214 discloses a device in which an OED emits light in the blue portion of the electromagnetic spectrum. This blue light is then passed through a fluorescent device so as to generate a RGB display. Likewise, U.S. Pat. No. 5,294,870 generates blue light from an OED, passing the same through an RGB fluorescent filter. Both types of devices, however, are limited by the fact that blue emitting organic electroluminescent displays have a limited thermal stability window, and poor efficiency. Moreover, devices such as those disclosed in the '214 patent appear to be characterized by limited lumous efficiency, i.e., a maximum on the order of approximately 2.55 foot lamberts per watt ("1 m/W").
In U.S. Pat. No. 5,705,285, issued Jan. 1, 1998, and entitled "Multicolored Organic Electroluminescent Display", an organic electroluminescent full color display by a combination of a blue/green emitting OED with pixelated blue, green absorption filters and red fluorescent filter is disclosed. This device overcomes some of the above problems but still leaves room for improvement.
Accordingly, there exists a need for a display device capable of reliably providing full color information thereon. The device should make use of a stable organic electroluminescent display element properly modified to provide red, green, and blue light as desired. Moreover, the display should be pixelated in order to assure high resolution information display.
The objective of the present invention is to further improve the performance of the full color OED display.